Modem mobile communication devices such as those based on 5G wireless networks require energy efficient wide bandwidth analog to digital converters (ADCs) to support processing of received radio signals. The fastest type of ADC are based on an architecture referred to as direct conversion or flash ADC. Direct conversion or flash ADC is preferred for wireless and mobile device applications because of the fast conversion times that can be achieved. A flash ADC uses a separate reference voltage and comparator for each quantization level resulting in 2N−1 reference voltages and comparators for an N bit ADC.
Sampling noise from multiple comparators can leak back onto the input voltage and/or reference voltages distorting the input and reference signals. This noise leakage, referred to as kickback noise, can significantly deteriorate the accuracy of a flash ADC.
Reference voltages may be created using a resistor string to divide a single voltage source. Preamplifiers are often inserted before each comparator to help prevent kickback noise from distorting the input and reference voltages. However, preamplifiers increase power consumption and can significantly reduce the battery life of a mobile communication devices. The preamplifiers also increase complexity and area of the ADC.
Power consumption of flash ADCs can be reduced by using floating voltage sources to provide the reference voltages. One approach to creating a floating voltage source is to use a switched capacitor network to shift the input voltage up or down by a predetermined amount. With this method no additional preamplifier stages are required and fully differential inputs are easy to implement. However, kickback transients from the comparators are not isolated and can significantly distort the input signal.